1. Field of the Invention
The present invention relates to a paper feeding device and a paper curling correcting device in an image forming apparatus such as a copying machine, a PPF, a page printer or the like.
2. Description of the Prior Art
In an image forming apparatus such as a copying machine, a PPF, a page printer or the like, a pair of paper separation rollers for preventing paper sheets from being fed in an overlapped state to feed a single paper sheet to a registration roller provided in a paper conveying path from a paper feeding cassette.
The pair of paper separation rollers comprises one forward roller and one reverse roller, as is well known. The forward roller is rotated and driven in the direction in which paper sheets are fed from the paper feeding cassette to the registration roller (hereinafter referred to as the direction of paper feeding). The reverse roller is rotated and driven in the direction in which paper sheets are returned from the registration roller to the paper feeding cassette (hereinafter referred to as the direction of paper return). The reverse roller is provided with a torque limiter for releasing the transmission of power from a driving source to the reverse roller when the load torque exerted on the reverse roller is not less than a predetermined regulated torque.
When a plurality of paper sheets are fed in an overlapped state to the pair of paper separation rollers, the load torque exerted on the reverse roller is lower than the regulated torque, so that the reverse roller is rotated and driven in the direction of paper return. Accordingly, the paper sheets on the reverse roller side are returned by the reverse roller, while a single paper sheet is fed to the registration roller by the forward roller.
When one paper sheet is fed to the area between the pair of paper separation rollers, the load torque exerted on the reverse roller is higher than the regulated torque, so that the transmission of power from the driving source to the reverse roller is released. Accordingly, the reverse roller is rotated in the direction of paper feeding in synchronism with the forward roller. Consequently, the paper sheet is fed to the registration roller.
In this type of image forming apparatus, the registration roller is driven in a state where a paper sheet is deflected between the registration roller and the pair of paper separation rollers so as to regulate the paper sheet straight in the direction of paper feeding and feed the same to a photosensitive drum. That is, the registration roller is driven after a delay of several milliseconds from the time when the forward end of the paper sheet passing between the pair of paper separation rollers abuts on the registration roller.
In the above described conventional device, the force of paper feeding produced by the pair of paper separation rollers depends on only the friction force between the paper sheet and the forward roller, so that the amount of deflection varies depending on the quality and the thickness of the paper sheet. Accordingly, the paper sheet may, in some cases, continue to be fed even if the amount of deflection of the paper sheet is saturated, so that the paper sheet is wrinkled.
Furthermore, examples of image forming apparatuses include an image forming apparatus in which a paper conveying path from a paper feeding cassette to a registration roller is formed in a U shape for the purpose of, for example, miniaturization. In such an image forming apparatus, however, paper sheets are curled to correspond to the curve of the paper conveying path in a U shape.
Further, examples of image forming apparatuses include an image forming apparatus for feeding parallel paper sheets such as envelopes contained in two rows in a paper feeding cassette by a paper feeding roller and conveying two paper sheets fed in parallel to a registration roller by passing through a pair of paper separation rollers or directly without passing through the pair of paper separation rollers. In such a image forming apparatus, the paper sheets contained in two rows in the paper feeding cassette may, in some cases, start to be fed in a state where the paper sheet contained in one of the rows in the paper feeding cassette is projected forward in the direction of paper feeding from the paper feeding cassette due to the feeding of the paper sheets in the row continuously or one over the other. Consequently, the paper sheet in the row projected forward in the direction of paper feeding from the paper feeding cassette abuts the registration roller prior to the paper sheet in the other row. In such a case, the pair of paper separation rollers or the paper feeding roller for conveying paper sheets to the registration roller continues to feed the paper sheets in a time period elapsed from the time when the paper sheet in the row projected forward in the direction of paper feeding from the paper feeding cassette abuts the registration roller until the paper sheet in the other row abuts the registration roller. Accordingly, the deflection of the paper sheet in the row which first abuts on the registration roller becomes large, so that the paper sheet is wrinkled or folded causing vibration and noise.
A first object of the present invention is to provide a paper feeding device capable of regulating a paper sheet straight in the direction of paper feeding as well as preventing the paper sheet from being wrinkled with respect to a plurality of types of paper sheets.
A second object of the present invention is to provide a paper curling correcting device for preventing a paper sheet from being curled in an image forming apparatus having a paper conveying path in a U shape.
A third object of the present invention is to provide a paper feeding device capable of preventing, when paper sheets are fed in parallel, one of the paper sheets which first abuts on a registration roller from being wrinkled.